1. Field of the Invention
This invention generally relates to explorations for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this invention relates to highly localized borehole investigations employing the introduction and measuring of individual survey currents injected into the wall of a borehole by capacitive coupling of electrodes on a tool moved along the borehole with the earth formation.
2. Background of the Art
Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a measure electrode (current source or sink) are used in conjunction with a diffuse return electrode (such as the tool body). A measure current flows in a circuit that connects a current source to the measure electrode, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present invention belongs to the first category.
Techniques for investigating the earth formation with arrays of measuring electrodes exist. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Pat. No. 685727 to Mann et al. U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. U.S. Pat. No. 6,348,796 to Evans et al., having the same assignee as the present invention and the contents of which are incorporated herein by reference, discloses an apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a focusing electrode. The focusing electrode is maintained at a slightly higher potential than the measure electrodes. A modulated electrical current with a carrier frequency of 1 MHz is injected into the formation. When used with a non-conducting fluid, capacitive coupling between the electrode and the conductive formation is provided by the dielectric of fluid. When used with a conducting borehole fluid, an additional capacitor may be incorporated into the circuit. The current in the measure electrode is indicative of the conductivity of the formation.
In oil-based muds, unfocused methods provides an alternative to the focused methods such as the device of Evans. In such devices, the electrodes are voltage measuring electrodes on a non-conducting pad. A current source and a current return provide a current flow in the formation parallel to the pad and voltage differences between electrodes are indicative of the formation resistivity. See, for example, U.S. Pat. No. 6,191,588 to Chen and WO2005/006023 of Cheung et al. With such a device, the problems caused by variations in standoff of the individual electrodes from the borehole wall are substantially eliminated. We refer to such devices as “four-terminal” devices and the corresponding methods as “four-terminal” methods.
However, because the four-terminal methods measures the formation resistivity with electrodes arranged in a direction parallel to the borehole wall, the image will depend on the direction of measurement for a layered or other inhomogeneous formation. The conventional resistivity imaging tools like that of Evans which measures the radial formation resistivity does not have this limitation. Specifically, if the bedding plane is inclined to the plane defined by a row of electrodes in an unfocused device, the measured resistivity will be a combination of the horizontal and vertical resistivity (defined here as parallel to and perpendicular to the bedding plane).
FIG. 2 (prior art, from Evans) illustrates an exemplary array of measure electrodes 115a, 115b, 115c . . . set within a substantially rectangular guard electrode 103 with gaps 107a (that contain insulating material therein). The guard electrode 103 is separated from the pad or body 101 by a substantially rectangular insulating gap 107b. In one embodiment of the invention, the spacing between the measure electrodes is selected as to provide overlap in azimuth and depth, i.e., the diameter D of the measure electrode is greater than the horizontal spacing d1 of the electrodes 115b, 115c in adjacent rows and the vertical spacing d2 between the rows of electrodes. In another embodiment of the invention, the electrodes do not have this azimuthal and vertical overlap, but due to a broadening of the measure beam used in Evans, overlap in azimuth and borehole depth of the region of investigation is obtained.
FIG. 3 depicts a borehole 121 penetrating a dipping-bed formation. The dipping beds are denoted by 123. When a focused imaging tool such as that of Evans is conveyed in the borehole, the tool should read the horizontal resistivity of the formation (the resistivity parallel to the bedding plane) when the pad surface is normal to the y-axis. This is because the focused currents travel radially into the formation, i.e., along the y-axis and parallel to the bedding. If, on the other hand, the pad surface is normal to the x-axis, the radial currents from the borehole will cut across the bedding plane and will be effected by both horizontal and vertical formation resistivities. However, since the current flowing into the formation is governed mainly by the conductive beds, the resistivity parallel to bedding again controls the measurement made by the focused imaging tool except at very steep dips. At intermediate pad face angles (between the x- and y-axes, both horizontal and vertical resistivities will affect the measure current of the tool.
The present invention is directed towards a method and apparatus that are relatively insensitive to the formation dip and may be used with an unfocused tool within a borehole, and may also be used with oil-based mud.